The control limits are calculated from the data. In a Control Chart, they are displayed as horizontal lines. For normally distributed data, they are an estimation of 3 standard deviations using time order information:
- Upper Control Limit (UCL) = Mean + 3 Standard Deviations
- Lower Control Limit (LCL) = Mean – 3 Standard Deviations
Attention: Control limits should not be mixed up with Specification Limits: Specification Limits say what the process variation should be (voice of the customer), whereas Control Limits say what the process variation actually is (voice of the process).
In the Control Phase of your DMAIC project, you might want to recalculate the control limits of your – hopefully – improved process. However, there are three prerequisites to fulfill, before you are allowed to do so:
- A change in the process can be proved based on statistical evidence (Nelson Rules)
- You have determined why the change occurred (in the best case due to your improvements)
- You are confident the process will stay changed. Call the new limits temporary until you get at least 24 new data points